Nitric Acid — Part 1: Laboratory preparation and Ostwald’s process: visual lesson, ICSE Class 10 Chemistry (Science Paper 2)

Lesson

Aqua fortis: strong water

Part 1 — Nitric acid, HNO₃, was once called aqua fortis (strong water) because it attacks most metals. It is a strong acid and a strong oxidising agent, and it is made in huge amounts for fertilisers and explosives.

In this lesson you will

Write the equation for preparing nitric acid from potassium or sodium nitrate
Describe the all-glass apparatus and explain the precautions taken
Explain why concentrated nitric acid often looks yellow
Write the equations, with conditions, for Ostwald’s process

Table of the formula, appearance and uses of nitric acid
Property	Nitric acid
Formula	HNO₃ (monobasic)
Pure acid	Colourless, fuming liquid
Laboratory concentrated acid	Often pale yellow
Main uses	Fertilisers such as ammonium nitrate, explosives, dyes

Concept

Laboratory preparation

Potassium nitrate (or sodium nitrate) is heated gently with concentrated sulphuric acid. Sulphuric acid is less volatile (higher boiling) than nitric acid, so it drives the more volatile nitric acid out as vapour, which is then condensed.

With sodium nitrate: NaNO₃ + H₂SO₄ → NaHSO₄ + HNO₃.

\ce{KNO3 + H2SO4 -> KHSO4 + HNO3}

Below 200 °C

Diagram

An all-glass apparatus

A glass retort with a glass stopper holds the mixture.
The retort neck leads into a receiver kept cold with running water.
No cork or rubber: nitric acid vapour attacks them.
Heat gently, keeping the temperature below 200 °C.

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